Coherent coupling between single quantum objects is at the heart of modern quantum physics. When coupling is strong enough to prevail over decoherence, it can be used for the engineering of correlated quantum states. Especially for solid-

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics 1 , nanoscale magnetometry 2,3 and biolabelling 4 . Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds 5 and milled nanodiamonds 6 , they have not been observed in very small isolated nanodiamonds 7 . Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.Detonation nanodiamond is routinely produced on an industrial scale, and the raw material can be disintegrated into a stable 5-nm monodisperse colloid 8 . The combination of inert core and chemically reactive surface, which can host a variety of moieties, is appealing for chemists, biologists and material scientists 9,10 . Quantum magnetometry 2,3 is an example of an emerging technology that will directly benefit from the availability of nanocrystals with welldefined sizes in the 5-nm range, because the sensitivity to single spins is inversely proportional to the cube of the distance between the sensor (that is, the nitrogen-vacancy (NV) centre) and the spin being detected.Producing and detecting NV colour centres in isolated 5-nm detonation nanodiamond has been controversial, and there has been some scepticism regarding their stability as a useful emitter in a discrete crystal. For example, theoretical calculations of the crystal energy budget favour the location of nitrogen on the surface rather than in the core, which seems to explain the limited observation of NV centres in chemical vapour deposition and high-pressure high-temperature grains of less than 40 nm in size 11,12 , and favours the prediction that nanodiamonds smaller than 10 nm in size do not contain NV centres 7,13 . Although sub-10-nm nanodiamonds with NV centres have been produced using a top-down approach (milling luminescent high-pressure hightemperature microdiamonds into 7-nm particles 6,14 ), the question of NV stability in isolated detonation nanodiamonds persists.In aggregated detonation nanodiamonds (agglomerates and agglutinates 8 ), high-sensitivity, time-gated luminescence and electronic paramagnetic resonance spectroscopy have been used to extract a weak NV signal from a strong luminescence background 5 . The experiments highlight the eclipsing nature of the graphitic surface layers in nanodiamond aggregates-NV centres were simply not visible through the broadband luminescence from the surface and grain boundary material. To distinguish the NV spectral signature from the large grain boundary luminescence overhead, diamond synthesis yielding discrete sub-10-nm detonation nanodiamonds is vital. Here, we use a robust deaggregation and dispersion method, which diminishes the crystal-crystal interaction to...

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication 1 . However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials 2,3 , particularly diamond 4-6 , however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin-orbit coupling. Here we show that quantum logic elements can be realized by exploring long-range magnetic dipolar coupling between individually addressable single electron spins associated with separate colour centres in diamond. The strong distance dependence of this coupling was used to characterize the separation of single qubits (98±3 Å) with an accuracy close to the value of the crystal-lattice spacing. Our demonstration of coherent control over both electron spins, conditional dynamics, selective readout as well as switchable interaction should open the way towards a viable room-temperature solid-state quantum register. As both electron spins are optically addressable, this solid-state quantum device operating at ambient conditions provides a degree of control that is at present available only for a few systems at low temperature.One of the greatest challenges in quantum information technology is to build a room-temperature scalable quantum processor 7 . Isolated electron and nuclear spins in solids are considered to be among the most promising candidates for qubits in that respect 3,8 . Several benchmark experiments including entanglement and elements of quantum memory 9 have been achieved with spin ensembles, but ultimate functionality requires encoding quantum information into single spins. This however creates serious challenges in readout, addressing and nano-engineering single-spin arrays. The availability of photon-assisted single-spin readout 10,11 and the possibility to create single defects by ion implantation 12,13 make nitrogen-vacancy defects in diamond one of the most promising candidates in this respect. Paramagnetic nuclei in the vicinity of the electron spin can be used as auxiliary qubits with even more favourable relaxation properties 14 . As a consequence, coherence between electron and nuclear spin qubits has been exploited for showing all basic elements of a room-temperature quantum register 5,[15][16][17] . The size of these registers however is limited to a few quantum bits owing to the limited number of nuclear spins that can be addressed in frequency space 15,18 . A critical step towards scalability is to develop a technique enabling mutual coupling of individual optically addressable quantum systems. The system used in this study is a pair of single electron spins associated with separate nitrogen-vacancy defects in diamond. A single defect consists of a substitutional nitrogen atom in the diamond lattice and an adjacent vacancy (Fig. 1a,b). The electron spin triplet ground state of the defect shows a spin-depen...

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Sensors based on the nitrogen-vacancy defect in diamond are being developed to measure weak magnetic and electric fields at the nanoscale. However, such sensors rely on measurements of a shift in the Lamor frequency of the defect, so an accumulation of quantum phase causes the measurement signal to exhibit a periodic modulation. This means that the measurement time is either restricted to half of one oscillation period, which limits accuracy, or that the magnetic field range must be known in advance. Moreover, the precision increases only slowly (as T(-0.5)) with measurement time T (ref. 3). Here, we implement a quantum phase estimation algorithm on a single nuclear spin in diamond to combine both high sensitivity and high dynamic range. By achieving a scaling of the precision with time to T(-0.85), we improve the sensitivity by a factor of 7.4 for an accessible field range of 16 mT, or, alternatively, we improve the dynamic range by a factor of 130 for a sensitivity of 2.5 µT Hz(-1/2). Quantum phase estimation algorithms have also recently been implemented using a single electron spin in a nitrogen-vacancy centre. These methods are applicable to a variety of field detection schemes, and do not require quantum entanglement.

Integrated optics provides an ideal test bed for the emulation of quantum systems via continuoustime quantum walks. Here we study the evolution of two-photon states in an elliptic array of waveguides. We characterise the photonic chip via coherent-light tomography and use the results to predict distinct differences between temporally indistinguishable and distinguishable two-photon inputs which we then compare with experimental observations. Our work highlights the feasibility for emulation of coherent quantum phenomena in three-dimensional waveguide structures.

We describe a new design for a q-wire with perfect transmission using a uniformly coupled Ising spin chain subject to global (homogeneously-applied) pulses. Besides allowing for perfect transport of single qubits, the design also yields the perfect "mirroring" of multiply encoded qubits within the wire. We further utilise this global-pulse generated perfect mirror operation as a "clock cycle" to perform universal quantum computation on these multiply encoded qubits where the interior of the q-wire serves as the quantum memory while the q-wire ends perform the quantum computation. We theoretically describe the operation of single and two-qubit quantum logic gates and show that only N − 1 complete mirror cycles are required to execute a quantum Fourier transform on N qubits encoded within the q-wire.The development of protocols for transmitting quantum states is a particularly important problem in quantum computation. The ability to produce q-wires would allow quantum information to be moved around within a quantum processor. In the initial work [1,2], the transport of quantum states through unmodulated spin chains was examined and less-than-perfect transport fidelities were found [1,3,4,5,6,7,8]. This is due to the dispersion of the quantum information along the chain [9]. Much work has since ensued searching for perfect q-wire transport schemes and briefly we can categorise these into: (1) if the nearest-neighbour couplings between systems comprising the q-wire are set to very specific values [7,8,10,11,12], one can achieve perfect transport.(2) One can achieve near perfect transport by encoding the quantum information into low-dispersion wavepackets, or by encoding/decoding via conditional quantum logic across multiple q-wires [3,9,13,14,15,16]. (3) Use 'gapped systems', where the q-wire ends are only weakly coupled to a strongly inter-coupled interior of the q-wire [17,18,19], to achieve near perfect transport. (4) Other possibilities include teleportation of the quantum information along the q-wire by measurements [20], encoding into soliton-like excitations [21], or use quantum cellular automata concepts [22,23]. Besides the transport of single qubits, of more interest is the capability of the q-wire to transport entire qubit registers via 'quantum mirror wires' [12,24]. Here an unknown multi-qubit quantum state, when encoded at one end of the wire is transmitted to the other end, but in reverse order,. As well as demonstrating that globally addressed qwires can yield perfect qubit transport and perfect multiqubit mirroring we will also show that they can be used to execute universal quantum computation. We achieve this via a combination of the application of selective local unitaries on the ends of the q-wire and homogenous local unitaries (HLUs [28]), (or global pulses), on the entire q-wire. The use of HLUs alone to perform quantum computation has been examined by a number of authors [2,29,30,31,32]. In all but the last of these, the application of HLUs alone is not sufficient to implement universal quant...

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